1. Technical Field
The present invention relates to an image forming apparatus which transfers an image formed on an image carrier onto a sheet, and more particularly to improvement effectively applied to an image forming apparatus that forms color images and duplex images.
2. Related Art
In an intermediate transfer type image forming apparatus, which is described here as an example of a conventional image forming apparatus, developing devices, one for each component color of black (Bk), yellow (Y), magenta (M) and cyan (C), are arranged around a latent image carrier such as a photosensitive drum, and a belt-like intermediate transfer member is arranged opposed to the latent image carrier, whereby unfixed toner images of these component colors formed on the latent image carrier are successively transferred onto the intermediate transfer member, on color image for each rotation of the latent image carrier, and overlapping primary transfer images on the intermediate transfer member are transferred onto a sheet, such as paper or OHP sheet, to form a desired image on the sheet (see the official gazette JP-A-5-323704 for example).
In this type of image forming apparatus, because the combined toner images transferred onto the intermediate transfer member are transferred en masse onto the sheet, there is an advantage that disturbance of an image and color misalignment that may occur during multiple transfers can be effectively prevented with out having to consider the sheet thickness, surface characteristics and transport characteristics of sheet with respect to latent image carrier.
In this kind of intermediate transfer type image forming apparatus, a known method is employed which, for accurate multiple transfers of component color images, uses a reference mark on the intermediate transfer member and aligns a series of component color images with this reference mark position (for example, JP-A-8-146698).
In order to improve productivity of the image forming apparatus described above, a method has already been proposed which provides plural image carrying regions on the intermediate transfer member and which, in a series of image making cycles up to the image transfer onto the sheet, has the intermediate transfer member carry and transfer the plural images onto the sheet according to predetermined timings determined by a timing decision means (for example, official gazette JP-A-8-314232).
This kind of intermediate transfer type image forming apparatus, however, has a technical problem that because a series of color overlapping processes (primary and secondary transfer processes) are performed at the same position at all times, the resistance of that portion of the intermediate transfer member which undergoes the color overlapping processes locally degrades, causing a significant local deterioration of the intermediate transfer member and making the transfer performance uneven.
In the type where plural image carrying regions are provided on the intermediate transfer member, in particular, the image carrying regions may not be used uniformly depending on the number of sheets and the sheet size used in each job. Normally, the first image carrying region is more frequently used than the remaining image carrying regions located behind, making local deteriorations of the first region more conspicuous.
Further, in the type where plural image carrying regions are provided on the intermediate transfer member and where a duplex mode is performed, if one of the image carrying regions is fixedly assigned to carry an image for the first side of the sheet and another region is fixedly assigned to carry an image for the second side of the sheet to improve productivity, it has been found that the region of the intermediate transfer member for carrying a second-side image is more conspicuously degraded.
This may be explained as follows.
The phenomenon in which, during the duplex mode, the surface resistance of a region of the intermediate transfer member for carrying the second-side image tends to decrease is considered due to the following reasons: (1) when paper or sheet with the first-side image already transferred thereon passes through a fixing device, the water content in the paper decreases to raise the resistivity of the paper; (2) as the resistivity of the paper increases, the transfer voltage for the second-side image must be increased to that extent from the voltage used for the transfer of the first-side image, with the result that an electric discharge between the paper and the intermediate transfer member becomes intense; and (3) the electric discharge makes the surface resistance of the intermediate transfer member more likely to decrease.
Further, during the process of transferring a second-side image, the sheet with the first-side image already fixed thereon comes to the secondary transfer section again. Normally, in the fixing device a toner release agent (for example, silicone oil) is supplied to the fixing member (e.g. fixing roll) and then in transferred to the sheet, from which it is further transferred locally onto the intermediate transfer member.
When the toner release agent is locally transferred onto the intermediate transfer member, the surface energy of that portion of the intermediate transfer member to which the release agent adheres changes, affecting the transfer performance of a toner image.
At this time because the tone release agent is not transferred to the image portion, the transfer efficiency of the non-image portion (release agent adhering portion) becomes higher than that of the image portion (release agent non-adhering portion).
In such a state, when a wide-area half-tone image is produced in a simplex mode, for example, because the transfer efficiency of a portion corresponding to the image portion in the automatic duplex mode is lower than other portion, the area corresponding to the image portion becomes a light half-tone image and the area corresponding to the non-image portion becomes a deep half-tone image. This phenomenon represents a technical problem of chronic density variation (transfer variation) leading to a possible serious image defect.
In the conventional intermediate transfer type image forming apparatus, a technique is proposed in which plural reference marks with different reflectivities and shapes are provided on the intermediate transfer member and that the reference position is changed for each series of color overlapping processes (see the official gazette JP-A-8-146698).
Although this technique of changing the reference position on the intermediate transfer member for each series of color overlapping processes may reduce the local degradation of the intermediate transfer member to some extent, it gives rise to a technical problem of a reduced productivity during the continuous printing operation. Further, in a type where plural image carrying regions are provided on the intermediate transfer member, the above technique does not give any suggestion as to how the reference position of each image carrying region can be changed.
The technical problems mentioned above are not limited to the intermediate transfer type image forming apparatus but can also occur with a direct transfer type image forming apparatus which transfers an image from a latent image carrier, such as a photosensitive belt, directly onto a sheet.